If you want to poke at this a bit, one way could be to test what sort of interferences disrupt different activities for you, compared to a friend.
I'm thinking of the bit in "Surely you're joking" where Feynman finds that he can't talk and maintain a mental counter at the same time, while a friend of his can -- because his friend's mental counter is visual.
Neat. I can do it both ways... actually, I can name at least four different ways of counting:
"Raw" counting, without any sensory component; really just a sense of magnitude. Seems to be a floating-point, with a really small number of bits; I usually lose track of the exact number by, oh, six.
Verbally. Interferes with talking, as you'd expect.
Visually, using actual 2/3D models of whatever I'm counting. No interference, but a strict upper limit, and interferes with seeing - well, usually the other way around. The upper limit still seems to be
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